rtlwifi: fix the wrong size to calculate fifo space

Give correct fifo size to calculate fifo space. Fortunately, the values of
RTL_PCI_MAX_RX_COUNT and TX_DESC_NUM_92E are the same in old code, so it
still works.

Signed-off-by: Steven Ting <steventing@realtek.com>
Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>